In the control of internal-combustion engines, systems are being provided in which the fuel-quantity control is intended to provide a requisite engine torque as a function of engine speed and accelerator pedal position. More specifically, a torque map is stored in memory as a function of engine speed and accelerator position for thereby regulating fuel quantity with the intent of providing the requisite torque. While such a system may function well for a given set of engine operating conditions, variables such as engine wear and fuel characteristics can interfere with the proper functioning of the system. For instance, with increasing investigation being placed upon the use of alternative fuels for powering internal-combustion engines, it is possible for the energy-producing characteristics of various fuels to differ greatly. Assuming the torque map for an engine is developed using a specific fuel as a standard, some alternative fuel may deliver substantially less energy for the same volumetric quantity of fuel delivered. While it might be possible to compensate for such variations by requiring the vehicle operator to adjust the positioning of an accelerator pedal, it is preferable that a particular accelerator-pedal position be capable of delivering a predetermined level of torque at a certain speed to afford repeatability to the operator and the vehicle. To accomodate the aforementioned need, it is contemplated that a signal indicative of actual engine torque might be compared with desired values of torque and the difference used as an adjustment or compensation factor in a fuel-quantity control loop. Of course, the provision of such an actual torque signal may have additional utility as well for the performance of other engine control and/or diagnostic functions.
Although commercial-available torque transducers do exist and at least theoretically might be used for obtaining a direct measurement of the output torque of an internal combustion engine, their actual use for so-called "on-line" or "on-board" measurement of torque would normally be precluded by cost and/or by packaging considerations. Such torque sensing units usually require separate shaft members and/or slip-ring couplings or rotary transformers for transmission of excitation and output signals from the rotating shaft and are thereby not readily adaptable to existing engine/transmission configurations.
Various techniques have been developed for providing some measure of engine torque using values derived from sensed measurements of engine speed. While these systems may avoid some of the aforementioned shortcomings associated with sensors which detect torque directly, they also possess their own limitations. For instance, in U.S. Pat. No. 4,169,371 to Witschi et al there is disclosed an arrangement for determining the torque of a drive system in dynamic operation, however, that system relies upon the acceleration speed of an unloaded engine and is of value principally for diagnostic and/or analytical purposes.
A more recent patent, U.S. Pat. No. 4,301,678 to Full et al and assigned to the assignee of the present application, discloses an analytical and diagnostic system for determining relative power contributions of each cylinder in an internal-combustion engine. That system is designed for use with an engine which is undergoing tests and is connected to a load such as a dynamometer. Firstly, it senses sub-cyclic speed fluctuations at both the engine and the load. Those sub-cyclic speed fluctuations are used to develop respective sub-cyclic torque signals for the engine is the load. A net sub-cyclic torque signal is then developed by determining the difference between the two aforementioned sub-cyclic torque signals. Sub-cyclic fluctuations in the net-torque signals are identified and compared to provide signal indications of the relative power contribution between cylinders. Although such system is advantageous for determining the relative health or torque contributions of the respective cylinders in an engine, it does not provide an engine torque signal which is suited to the continuous on-line control of the engine.